1. Field of the Invention
The present invention relates generally to reconstitutable fuel assemblies for nuclear reactors and, more particularly, is concerned with an arrangement and method for attaching and reattaching a top nozzle in a reconstitutable fuel assembly.
2. Description of the Prior Art
Conventional designs of fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend above and below the opposite ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in openings provided in the top nozzle. Conventional fuel assemblies also have employed a fuel assembly hold-down device to prevent the force of the upward coolant flow from lifting a fuel assembly into damaging contact with the upper core support plate of the reactor, while allowing for changes in fuel assembly length due to core induced thermal expansion and the like. Such hold-down devices have included the use of springs surrounding the guide thimbles, such as seen in U.S. Pat. Nos. 3,770,583 and 3,814,667 to Klumb et al and 4,269,661 to Kmonk et al, and in the first patent application cross-referenced above.
Due to occasional failure of some fuel rods during normal reactor operation and in view of the high cost associated with replacing fuel assemblies containing failed fuel rods, the trend is currently toward making fuel assemblies reconstitutable in order to minimize operating and maintenance expenses. Conventional reconstitutable fuel assemblies incorporate design features arranged to permit the removal and replacement of individual failed fuel rods. Reconstitution has been made possible by providing a fuel assembly with a removable top nozzle. The top nozzle is mechanically fastened usually by a threaded arrangement to the upper end of each control rod guide thimble, and the top nozzle can be removed remotely from an irradiated fuel assembly while it is still submerged in a neutron-absorbing liquid. Once removal and replacement of the failed fuel rods have been carried out on the irradiated fuel assembly submerged at a work station and after the top nozzle has been remounted on the guide thimbles of the fuel assembly, the reconstituted assembly can then be reinserted into the reactor core and used until the end of its useful life.
One recently proposed design for a reconstitutable fuel assembly top nozzle is described and illustrated in the second patent application cross-referenced above. The proposed top nozzle overcomes certain problems associated with the removable top nozzle designs of the above-cited U.S. patents and first cross-referenced application. Specifically, it includes improved structures which eliminate relative sliding engagement between the upper core support plate and the hold-down structure of the top nozzle while providing removable mounting of the top nozzle as a unitary subassembly on the guide thimbles of the reconstitutable fuel assembly. The fourth cross-referenced application builds on the highly satisfactory design of the second application by providing an integral reusable locking arrangement for the removable top nozzle which requires no special tooling to actuate it and, as a result, greatly reduces the complexity of the fuel assembly reconstitution operation.
While the proposed removable top nozzle designs of the second and fourth cross-referenced patent applications, as just briefly described, were considered to be highly satisfactory solutions to the problems existing previously with regard to earlier designs, it has been recently recognized that even these do not provide optimum solutions to some of the earlier as well as other problems. Specifically, it has been found that protection of the individual hold-down springs of the top nozzle is inadequately provided for by the designs of either of the second and fourth cross-referenced applications. Further, in view that a fuel assembly typically will not be reconstituted more than two times (one time being a realistic estimate), the reusable looking arrangement of the fourth application is now considered to involve fabrication costs which are too high.
Consequently, a need exists for a different approach to spring protection from coolant cross flow and to top nozzle attachment and reattachment, one with the objective of retaining the beneficial design features of the top nozzle disclosed in the cross-referenced applications while avoiding their short-comings.